We concentrate on the critical distinction between immortal early embryonic cells and mortal differentiating derivative cells. Our approach is to optimize the use of systematic genomic approaches for profiling gene expression patterns by large-scale cDNA sequencing and cDNA microarray technology with a current cohort of ~22,000 genes collected from early mouse embryos and stem cells. We are particularly focusing on preimplantation development - the process of fertilized egg gradually losing its totipotency. In our previous work, we identified many genes that show stage-specific expression patterns during preimplantation mouse development. However, these genes have been identified by EST frequency, which is a relatively inaccurate and far from ideal way to do gene expression profiling. The cDNA microarray-based gene expression profiling will provide more reliable information. To this end, we have been working on large-scale gene expression profiling of each stage of preimplantation mouse development. To start to examine whether these genes identified here might be involved in the immortal/mortal transition process, we are currently analyzing the nature of these genes in greater detail. We are also testing various antisense oligonucleotides and siRNA to inhibit the function of specific gene in preimplantation mouse embryos. Finally, we are also developing a high throughput whole-mount in situ hybridization technique on preimplantation embryos.